1. Field of the Invention
This invention relates to a bending simulation method for automatically obtaining a stroke of an optimum punch for obtaining a target bending angle by simulation without actual trial bending by a bending machine and an apparatus therefor.
2. Description of the Prior Art
FIG. 1 is a schematic structure diagram of a production line control system for bending machines. The production line control system for the bending machine shown in FIG. 1 comprises a centralized station 1, a CAE apparatus 2, an automatic programming apparatus 3 and an NC apparatus 5 for controlling the bending machine 4, these components being connected through LAN.
Generally, in V-shape bending, spring-back occurs when bending load is released. Thus, in the production line control system for the bending machine shown in FIG. 1, when an operator instructs a bending elongation amount for a position to be bent by an automatic programming apparatus 3, he enters workpiece condition, tool condition, target Di value which is a stroke amount of punch and the like (hereinafter generally referred to as bending information) shown in FIG. 2 through the CAE apparatus 2, so as to carry out simulation of sheet material formation considering the spring-back.
As shown in FIG. 2, the workpiece condition includes sheet thickness, Young's modulus of elasticity, Poisson's ratio, yield strength, processing hardness index and the like. The tool condition includes die groove width Vd, die radius, die groove angle, punch tip radius, tip angle, punch width Vp and the like.
When, in a simulation screen of the automatic programming apparatus 3, a deformation angle of workpiece sectional image after a spring-back reaches a target folding angle (hereinafter referred to as target angle), the operator inputs the bending information such as the target Di value, bending elongation amount, processing load and the like.
Then, according to this filed bending information, V-shape bending NC program is produced and transmitted to the centralized station 1 or NC machine so as to start processing.
In this simulation calculation used in this CAE apparatus 2, generally workpiece deformation shape after the spring-back is obtained by using elastoplasticity finite-element method (hereinafter referred to as finite-element method) capable of treating elastic restoration when loading weight is released.
As shown in FIG. 3A, the automatic programming apparatus 3 displays the workpiece sectional image 11, the die sectional image 12 and the punch sectional image 13. And until the target Di value is reached, it descends the punch sectional image 13 and displays the workpiece sectional image 14 in which the workpiece sectional image is deformed as shown in FIG. 3B by the finite-element method. According to this finite-element method, mesh division (element decomposition) of workpiece is carried out so as to deform the image.
Next, when the punch sectional image 13 is descended to reach the target Di value, the workpiece deformation image after the spring-back is obtained by the finite-element method and displayed as shown in FIG. 3C.
If the workpiece shape after the spring-back is not formed at the target angle, the operator corrects and enters the initial target Di value so as to perform the previously described workpiece again.
However, the bending simulation according to the conventional finite-element method has such a problem that the number of workpiece steps which the operator must do increases, because if the workpiece deformation angle after the spring-back have not reached its target angle, the operator must calculate and enter the initial target Di value again.
The initial target Di value must be set taking an occurrence of the spring-back into account. Thus, to set new initial target Di value, a worker not skilled in V-shape bending and having no high level know-how cannot set the new initial target Di value easily.
Further, the conventional art has another problem that in the V-shaped bending, the actual target angle bending may not be attained depending on the conditions.
On the other hand, upon simulation, mesh division of dividing the workpiece into elements is carried out by an operator based on punch shape, workpiece shape and the like. This mesh division is often set with the same intervals.
However, because generation of the mesh is carried out by an operator, there are following problems which must be solved.
(1) In the case when the mesh intervals are the same, there is a problem that deformation of a contact point between the workpiece and the punch is different from the actual one if the contact point is insufficient.
(2) Even if a sufficient contact point is entered, the mesh at the same intervals has a problem that the calculation according to the infinite-element method takes a so long time.